Pitting is a major source of failure in gear and bearing devices. The repeated cycling of loads and changes in slide to roll ratios leads to the formation of cracks on stressed surfaces. These initial cracks propagate in micro-pitting. This micro-pitting then grows over time to form macro-pitting, which typically results in component failure. Such failure is addressed in the prior art by using precisely controlled surface roughness, lubricant and additives in lubricant, and use of high-purity metallic components, which is very costly.
Further, gear and bearing devices are being developed and applied to new technologies that present additional challenges. For example, wind energy is a promising and fastest growing power generation source that relies heavily on gear and bearing devices with specific reliability needs. An increase in the number of utility scale wind plants have increased the focus on the high operation and maintenance costs of wind turbines as these ultimately impact the cost of wind energy. The drive train and actuators of wind turbines are major sources of failures arising from the variability of wind, torque reversals, fluctuation in energy demands, misalignment, and harsh environment conditions. Bearings and gears in wind turbine drive trains suffer from failure modes like micropitting, scuffing, spalling, and smearing, although these elements were designed to meet twenty year service lives assuming that proper lubrication and maintenance practices, and especially no unusual loads were encountered. If a bearing has a low concentration of non-metallic inclusions in the steel, operates at the designed contact stress, and maintains an adequate lubricant film thickness in the contact, then end of service life will be due to sub-surface originated spalling. Surface originated fatigue or pitting is caused by surface or near surface stress risers such as non-metallic inclusions, plastically deformed material, martensite transformation products, or several other factors. A particular type of surface initiated fatigue is known as micropitting which is a common failure mode encountered by gears and bearings. Specifically, many main shaft spherical roller bearings in wind turbines are life limited due to spalls arising from micropitting wear. Micropitting is associated with the initiation and propagation of micro-cracks in the direction of traction forces. The progression of micro-pits alters the surface profile of a bearing raceway or gear tooth which generates regions of large stress concentrations. The increase in localized stresses leads to fatigue failure through the formation of macro-pits or spalls. Micropitting is affected by several factors such as lubricant type, contaminants, temperature, contact stresses, hardness, sliding speed, and surface roughness.
Studies were carried out over the last few decades to understand the mechanism of micropitting. According to Morales-Espejel and Brizmer, micropitting depends on the lubrication conditions and roughness of the contacting surfaces, the presence of slip (between 0.5 and 2%), and the associated boundary friction shear stress are required for the generation of micropitting. Oila and Bull suggested that contact stress has the greatest impact on micropitting initiation, while the progression of micropitting is affected mostly by speed and slide to roll ratio. Lubrication conditions are best quantified by the parameter lambda (A), which is the ratio of the lubricant film thickness to the square root of composite surface roughness. Operating temperature, viscosity, and operating speed all affect the lubricant film thickness and hence A. Brechot et al reported that oils with antiwear and extreme pressure additives that are used to prevent scuffing and wear can promote micropitting. Micropitting has proven to be difficult to eliminate through lubricant chemistry alone.
A number of solutions have been suggested to mitigate micropitting. Super-finishing is a process used on gear teeth to increase load bearing area and reduce the severity of asperity interactions in boundary lubrication (i.e., λ<1). Apart from super-finishing, other surface engineering techniques are also employed to reduce asperity contact and provide barriers to wear. Physical vapor deposition (PVD) coatings composed of nitrides, sulfides and carbides were examined for their ability to prevent micropitting. PVD coatings can be very effective at reducing or eliminating many wear modes. Among these coatings, diamond like carbon (DLC) coatings are now being used in numerous applications for wear resistant purposes due to their desirable tribological performance. DLC has been modified over the years to possess ultra-low friction and high wear resistance. DLC coatings can be doped or alloyed to increase their functionality. The properties (hardness, toughness, thermal stability) of DLC coatings are further increased by using novel coating architectures that consist of nanocrystalline precipitates and nanosized multilayers. Hydrogen-free DLC coatings deposited from solid carbon targets can be extremely hard, while hydrogenated DLCs are usually much softer. In this research, coatings having indentation hardness values greater than 10 GPa are referred to as hard coatings, while coatings with indentation hardness values less than 10 GPa are referred to as soft coatings. Precursor hydrocarbon gases such as methane and acetylene are typically used in the deposition of DLC that contain large amounts of hydrogen. Hard DLC have been shown to be very successful at mitigating many wear issues encountered by bearings and gears operation in boundary lubrication, including micropitting. Surface treatments such as black oxide and phosphate conversions are also applied to bearings and gears to address micropitting. These conversions are thick, sacrificial layers that work to rapidly break-in the surfaces of the components, reducing asperity contact, and delaying the onset of micropitting. Most of the studies reported on exploring the use of DLC to mitigate micropitting prevention were carried out with hard DLC coatings.